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Transcript of Rate Law Yuan
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Yuana Elly. A., S.T., M.ScSeptember, 104
Rate laws
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Rate law and Reaction orderThe order of a reaction refers to the
powers to which the concentrations are raised in the kinetic rate law.
the reaction in the above equation is α order with respect to reactant A. and β order with respect to reactant B.
The overall order of the reaction, n, isn = α + β
The units of –rA , are always in terms of concentration per unit time while the units of the specific reaction rate, kA, will vary with the order of the reaction.
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Consider a reaction involving only one reactant, such asA Products
with a reaction order n. The units of the specific reaction rate constant k are
Then for
time
ionconcentratk
n
1)(
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Rate equationSingle and multiple reaction :
Single reaction : A B or A + B C + DMultiple reaction :
Series reaction : A B CParallel reaction : A B
A C
Elementary and non elementary reactions :Elementary reactions : rate equation corresponds
to the stoichiometric reaction A + 2B C, -rA = k. CA. CB
2. where k : rate constantNon elementary reaction : rate equation isn’t
related to the stoichiometric reaction 2A 3B, -rA= k.C2
A/CB, where k : rate constant
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Elementary rate lawsFor an elementary reaction such as
the bimoIecuIar reaction between oxygen and methanol
The reaction is first order in molecular oxygen and first order in methanol therefore. we say both the reaction and the rate law are elementary.
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Types of Elementary ReactionUnimolecular
O* O + ORate = k. [O*]
BimolecularNO(g) + O3(g) NO2(g) + O2(g)Rate = k. [NO]. [O3]
TermolecularO + O2 + M O3 + M Rate = k. [O]. [O2]. [M]
Termolecular reactions are low probability reactions; require three species to come together simultaneously
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2H2O2(aq) 2 H2O(l) + O2(g)
Experimental rate law: Rate = k [H2O2][I-]
(i) H2O2 + I- H2O + OI- slow
(ii) OI- + H2O2 H2O + O2 + I- fast
Rate depends on the slow, rate-determining step, (i)
Here, OI- is the intermediate species
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2NO2 + F2 2 NO2F
Experimental rate expressionrate = kobs [NO2] [F2]
Possible mechanism which fits the experimental observationNO2 + F2 NO2F + F slow
NO2 + F NO2F fast
First step: rate = k1 [NO2] [F2] rate determining step
Second step: rate = k2 [NO2] [F]
The rate of the first reaction determines the rate of the overall reaction
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Elementary Rate LawsThere are many reactions where the stoichiometric
coefficients in the reaction are identical to the reaction orders but the reactions are not elementary owing to such things as path-ways involving active intermediates and series reactions. For these reactions that are not elementary but whose stoichiometric coefficients are identical to the reaction orders in the rate law, we say the reaction follows an elementary rate law.
For example, the oxidation reaction of nitric oxide
2NO + O2 2NO2
The reaction is not elementary but follows the elementary rate law
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Non elementary Rate Laws
A large number of both homogeneous and heterogeneous reactions do not follow simple rate laws.
Examples of Homogeneous ReactionsCO +Cl2 COCl2
This reaction is first order with respect to carbon monoxide, three-halves order with respect to chlorine. and five-halves order overall
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Non elementary Rate LawsExample of Heterogeneous Reactions In many pas-solid catalyzed reactions. It
historically has been the practice to write the rate law in terms of partial pressures rather than concentrations.
C6H5CH3+ H2 C6H6 +CH4
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